Hydrogen-selective silica-based membrane

ABSTRACT

A hydrogen permselective membrane, a method of forming a permselective membrane and an apparatus comprising a permselective membrane, a porous substrate and an intermediate layer are described.

CROSS-REFERENCE TO RELATED APPLICATIONS

Not Applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to inorganic membranes that arepermeable to small gas molecules. More particularly, the presentinvention relates to permeable membranes deposited on porous substrates,having a graded intermediate layer, that exhibit both a high hydrogenpermeance and a high hydrogen permselectivity.

2. Description of the Related Art

Permeable materials are those through which gases or liquids may pass.Membranes are one type of permeable material and are composed of thinsheets of natural or synthetic material. Frequently, membranes exhibitdifferent permeances—i.e., permeation rates—for different chemicalspecies. In this regard, permselectivity is the preferred permeation ofone chemical species through a membrane with respect to another chemicalspecies. Permselectivity of the desired permeate with respect to anotherchemical species is calculated as the ratio of the permeance of thedesired permeate to the permeance of the other chemical species.

Permselective membranes are promising in a variety of applicationsincluding gas separation, electrodialysis, metal recovery, pervaporationand battery separators. Recently, interest has developed in usingpermselective membranes in so-called membrane reactors, which allow thesimultaneous production and selective removal of products. One regime inwhich permselective membranes are particularly promising is that ofequilibrium-limited reactions. In such reactions, yields are reduced byreaction reversibility. Preferential removal of one or more of thereaction products effectively shifts the equilibrium—or, stateddifferently, decreases the rate of the reverse reaction—therebyovercoming thermodynamic limitations.

One example of an equilibrium limited reaction is the methanedry-reforming reaction [1]:CH₄+CO₂

2CO+2H₂ (ΔH°₂₉₈=247 kJ·mole⁻¹)  [1]This reaction provides a pathway to convert carbon dioxide, aproblematic greenhouse gas, and methane, a plentiful natural resource,into synthesis gas—i.e., a mixture of hydrogen and carbon monoxide.Synthesis gas is an industrially important feedstock that is used in thepreparation of ethylene glycol, acetic acid, ethylene, fuels and severalother commercially important chemicals. Unfortunately, the conversion ofmethane and carbon dioxide to synthesis gas is limited by thereversibility of the reaction—i.e., the ability of hydrogen and carbonmonoxide to regenerate methane and carbon dioxide. The yield can beimproved, however, by selectively removing one or both of the productsas they are formed. Doing so mitigates the extent of the reversereaction.

Other examples of equilibrium-limited reactions that produce hydrogengas are the decomposition of hydrogen sulfide [2] and ammonia [3]:H₂S

S(s)+H₂  [2]2NH₃

N₂+3H₂  [3]Hydrogen sulfide and ammonia are frequent and undesirable byproducts ofnumerous chemical reactions. Thus, reactions [2] and [3] offer anabatement technique for reducing the levels of these compounds. Like themethane dry-reforming reaction, the products of these reactions can befavored by removing hydrogen as it is produced. In short, hydrogenpermselective membranes offer the potential to overcome severalequilibrium-limited reactions in commercially useful ways.

Conventional hydrogen permselective membranes have typically beenprepared on porous Vycor™ glass or ceramic supports by sol-gel orchemical vapor deposition (CVD) methods. Generally, a thin silicamembrane can be directly coated or deposited on mesoporous supports suchas Vycor™ glass with 4 nm pore size, but cannot be placed directly onmacroporous supports with pore sizes substantially larger than 50 nm.Hwang et al. attempted chemical vapor deposition (CVD) oftetraethylorthosilicate (TEOS) on a porous alumina tube with pore sizeof 100 nm and obtained only a selectivity of 5.2 for the separation ofH₂ from N₂ at 873 K after 32 hours of deposition (G-J. Hwang, et al., J.Membr. Sci. 162 (1999) 83). Such low selectivities are indicative of thepresence of large pore defects.

Coating macroporous supports using an intermediate mesoporousgamma-alumina sol layer prior to the deposition of a silica membrane hasbeen attempted to overcome this problem of large pore defects. However,the quality of the sol layer is limited by, among other things, the sizedistribution of the sol particles. On the one hand, as depicted in FIG.1, when a dipping solution is used consisting of sol particles that arelarge compared with the pore size of the supports, the particles do notprovide additional restrictive passages for controlling selectivity.Additionally, they do not cover the surface uniformly and can leavepatches of exposed, untreated surface. On the other hand, as depicted inFIG. 2, if a dilute dipping solution is used consisting of sol particlesthat are small compared with the pore size of the supports, these smallsol particles do not easily form “bridges” over some of the largefeatures and extra large pores of the supports because of infiltrationduring dip-coating. Even if such “bridges” are formed, they are notstrong and are easily broken or cracked. This problem becomesincreasingly more serious for supports with broader pore sizedistributions.

Previous work in the literature describes a method of depositing agamma-alumina layer on a support, for example, (R. J. R. Uhlhom, et al.J. Mater. Sci. 27 (1992) 527). In this work the dipping-calciningprocedure is repeated at least 2-3 times by using a concentrated dippingsolution with a boehmite sol concentration of 0.5-1.0 M to obtain athick, defect-free gamma-alumina layer. The layer thickness of agamma-alumina supported layer, made with 0.6 M dipping solutioncontaining PVA, was typically 5-6 μm after three subsequent dippingsteps with 3 second dipping time. Thinner layers are preferred becausethicker layers decrease permeability for the desired permeate.

Although Uhlhorn, et al's method has been used in the past, not muchattention has been placed on the physical characteristics of theseintermediate mesoporous gamma-alumina sol layers. An ideal intermediatelayer would be thin, continuous, defect-free and exhibit a highpermeability for the desired permeate.

SUMMARY OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention include a method formaking permselective asymmetric membranes comprising a porousintermediate layer produced from gamma-alumina sols having a narrow,well-defined particle size distribution. In a preferred embodiment, thesols are mixed in a dilute solution and used to form the porousintermediate layer. Preferably, a plurality of sol solutions are used,whereby sols with increasingly smaller particle size distributions areused to create a graded porous intermediate layer. According to onepreferred embodiment, a silica layer is deposited on the gamma-aluminamembrane, thereby forming a composite membrane. The preferredembodiments of the present invention also include the permselectiveasymmetric membranes produced from these methods. The resultingpermselective asymmetric membranes preferably show high hydrogenselectivity and permeance.

The preferred embodiments of the present invention include boehmite solshaving a narrowly tailored particle size distribution and a method forproducing same.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more detailed description of the preferred embodiment of thepresent invention, reference will now be made to the accompanyingdrawings, wherein:

FIG. 1 is a schematic of a support with large sol particles placedthereon;

FIG. 2 is a schematic of a support with small sol particles placedthereon;

FIG. 3 is a schematic of a support with graded sol particles placedthereon, forming a uniform substrate layer, in accordance with apreferred embodiment;

FIG. 4 is a schematic of a composite membrane in accordance with apreferred embodiment, including a support, a uniform substrate layer,and a silica layer;

FIG. 5 is graph of particle size distributions of boehmite sols peptizedwith various acids;

FIG. 6 is a graph of particle size distributions of boehmite solspeptized with acetic acid;

FIG. 7 is a graph of particle size distributions of boehmite solshydrolyzed for various times;

FIG. 8 is a schematic of dip-coating machine for use in accordance witha preferred embodiment;

FIG. 9 is a graph of pore size distributions of gamma-alumina supportsobtained from boehmite sols having various particle sizes;

FIG. 10 is a schematic of a suitable CVD apparatus for use in thedeposition of the silica layer;

FIG. 11 is a graph of permeation properties at 873 K of an ungraded fourlayer silica/alumina membrane;

FIG. 12 is a graph of permeation properties at 873 K of a graded fourlayer silica/alumina membrane in accordance with a preferred embodiment;

FIG. 13 is a graph of permeation properties at 873 K of a graded fivelayer silica/alumina membrane in accordance with a preferred embodiment;

FIG. 14 is a graph of permeability of He, H₂, and Ne through a gradedfive-layer silica/alumina membrane in accordance with a preferredembodiment;

FIG. 15 is a low-resolution electron micrograph of a graded five-layersilica/alumina membrane prepared by the method described in Example 9;and

FIG. 16 is a high-resolution electron micrograph of the gradedfive-layer silica/alumina membrane prepared by the method described inExample 9.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention derives, in part, from the discovery that thin,high quality films with high permeance can be deposited on poroussubstrates using dilute sols of sequentially smaller particle sizedistribution. These films can be used as intermediate layers between aporous support and a membrane material to create asymmetric membranesexhibiting a combination of high hydrogen permeability and selectivity.

According to one preferred embodiment of the present invention, a poroussupport is dip-coated with a series of boehmite sols having sequentiallydecreasing particle size distributions. As depicted in FIG. 3, thisgives rise to a graded structure with satisfactory filling of voids,thereby avoiding (or at least minimizing) defects. In order toaccomplish this task, sols having controlled particles sizedistributions are needed.

Boehmite sols are poorly understood (V. V. Nazarov and O. B.Pavlova-Verevkina, Colloid Journal, 60 (1998) 738). The published dataon the relationship between the particle size distribution of boehmitesols and process parameters are very limited and contradictory to someextent, even when the same measuring techniques are used. For example,in a case where the same aluminum precursor and peptization agent wereused for synthesis, and the same quasi-elastic light scatteringtechnique was employed for analysis, Larbot et al. (A. Larbot, et al.,Key Eng. Mater., 61-62 (1991) 395) reported the median particle size ofa boehmite sol increasing from 100 to 250 nm with the pH valueincreasing from 3.4 to 4.2, while Xia et al. (C. Xia, et al. J. Membr.Sci. 116 (1996) 9) found the sol particle size decreasing considerablyfrom 470 nm to 144 nm and then slightly increasing to 228 nm with anincrease of pH from 3.22 to 4.42. Lijzenga et al. (C. Lijenga, et al.,Key Eng. Mater., 61-62 (1991) 383) obtained a boehmite sol with aparticle size in the range of 40-60 nm at a pH between 4 and 5.Zakharchenya (R. I. Zakharchenya, J. Sol-Gel Sci. & Tech. 6 (1996) 179)reported that the average particle sizes of the boehmite sols werealways in the interval of 20±15 nm when different aluminum precursors,peptization acids and molar ratio of H⁺/Alkoxide in the range of0.01-0.3 were employed. Yoldas (B. E. Yoldas, Ceram. Bull. 54 (3) (1975)289) prepared aluminum sols by the hydrolysis of aluminum alkoxideprecursors in various acid solutions and found that when HCl was usedmean particle size increased from 5-35 nm to 10-100 nm with increasingacid concentration.

The preferred embodiments of the present invention include methods forpreparing sols, preferably alumina sols, having tailored median particlesizes with narrow particle size distributions as well as methods forapplying the sols as a mesoporous layer, preferably a mesoporousgamma-alumina layer, onto macroporous supports. The following preferredembodiments for obtaining these sols are exemplary and are not intendedto be limiting.

Preparation of the Sols

A quantity of 0.2 mol of aluminum isopropoxide was added to 300 ml ofdistilled water at room temperature. The mixture was quickly heated to353 K within 0.5 hours with high speed stirring [˜500 rpm] and wasmaintained at this temperature for 24 hours, allowing hydrolysis of theisopropoxide, forming a precipitate. The precipitate was then heated to365 K and peptized using a predetermined quantity of acid. Various acidswere used, including acetic acid, nitric acid, hydrochloric acid. Thesolution was kept at 365 K with refluxing for 20 hours to get a stablesol. A stable sol is herein defined as a sol that retains its particlesize for three months to within 5% of its original particle size.

For some embodiments, the aluminum isopropoxide was hydrolyzed at 353 Kfor 24 hours and then peptized using acetic acid, nitric acid and/orhydrochloric acid, using a molar ratio of H⁺/Alkoxide=0.10 (FIG. 5). Itshould be understood that the H⁺ refers to the total normal equivalentsof H⁺ and not the hydronium ion concentration taking into account the Kaof the acid. For other embodiments, the aluminum isopropoxide washydrolyzed at 353 K for 24 hours and then peptized using acetic acidusing different molar ratios of H⁺/Alkoxide in the range of 0.03-0.25(FIG. 6). In still other embodiments, the aluminum isopropoxide washydrolyzed at 353 K for 0.5, 3, 24 and 72 hours, and then peptized usingacetic acid with a molar ratio of H⁺/Alkoxide=0.15 (FIG. 7). These solscharacterized in FIGS. 5-7 remained stable for more than 3 months.

A dynamic light scattering analyzer (Horiba Model LB-500) was used tomeasure the particle size distribution of the sols. The concentration ofthe sols obtained in each of FIGS. 5-7 was in the range of 0.75-0.85 M,and the exact sol concentration was obtained by measuring the volume ofthe sol. As can be appreciated from comparing FIGS. 5-7, stable boehmitesols with particle sizes in the range of 10-1000 nm were produced bycontrolling the sol-gel process parameters such as acid type, acidconcentration and hydrolysis time.

The procedures provided above are given as a general description and donot limit the invention in so far as the quantity and type of aluminumprecursor, which may be any hydrolyzable species or combination thereofincluding but not limited to aluminum methoxide, ethoxide, propoxide,isopropoxide, butoxide, or aluminum chloride. The acid may be anyinorganic acid or combination thereof including but not limited tonitric acid, hydrochloric acid, or perchloric acid, or any organic acidor combination thereof including but not limited to formic acid, aceticacid, propionic acid, benzoic acid, etc. Furthermore, the acid may be acombination of organic and inorganic acids. Additionally, times andtemperatures of hydrolysis, peptizing, and aging may deviateconsiderably from the above-described general method.

Preparation of the Intermediate Sol Layer

The preferred embodiments of the present invention employed commercialalumina membrane tubes (PALL Corporation, Membralox® TI-70-25Z MembraneTube, I.D.=7 mm, O.D.=10 mm) with a nominal pore size of 100 nm wereused as the support. The membrane tubes were cut into short piecesapproximately 3 cm in length, washed with hot water to remove aluminaparticles, and connected with non-porous alumina tubes (Degussit/Pascal,http://www.pascal-co-ltd.co.jp/PDFcatalog/D1.pdf) at both ends withceramic joints. The ceramic joints were made with a glaze (Duncan IN1001) fired at 1153 K for 0.5 hours.

Dilute dipping solutions were prepared by mixing the previouslydescribed boehmite sols with an aqueous PVA solution of concentration of3.5 wt % and diluting with enough distilled water to form 0.15 Mconcentrations (Al content) of the sol.

The alumina support was dipped into the dipping solution and withdrawnafter 10 seconds at a rate of 0.01 meter/second using a dip-coatingmachine, as shown in FIG. 8. The dip-coated alumina support was dried inambient air for 24 hours, and then placed in a furnace connected with atemperature controller. The support was heated to 923 K in air at a rateof 1 K/minute and calcined in air at 923 K for 2 hours. As describedfurther below, the dipping-calcining process was repeated 3-5 times persupport using either a single sol or a series of sols havingsequentially smaller average particle sizes to form a uniform membranesubstrate.

EXAMPLES OVERVIEW

In the following examples, four boehmite sols prepared in accordancewith the above methods having median particle sizes of 630, 200, 55 and40 nm were used to prepare mesoporous gamma-alumina layers on poroussupports according to the above procedures. For each of the supports,the diameter of the pores was much larger than molecular sizes (˜0.2 nm)and the untreated supports did not provide selectivity for hydrogen orother gases.

In a first comparative example, a gamma-alumina layer was prepared on a100 nm pore size alumina support by dipping-calcining the alumina fourtimes in the same sol containing particles of size 55 nm. In anothercomparative example, a gamma-alumina layer was prepared on a 100 nm poresize alumina support by dipping-calcining the alumina four times in thesame sol of particle size 630 nm. In another example, a gamma-aluminalayer was prepared on a 100 nm pore size alumina support bydipping-calcining the alumina successively in sols in the order ofdecreasing particle size: 630, 630, 200 and 40 μm. In another example, agamma-alumina layer was prepared on a 100 nm pore size alumina supportby dipping-calcining the alumina in sols of decreasing particle size:630, 630, 200, 40 and 40 nm.

The microstructures of the gamma-alumina layers were characterized bynitrogen physisorption conducted in a volumetric unit (MicromeriticsASAP 2000). The alumina layer samples were prepared using the sameprocedure and parameters as the supported membranes. First, a boehmitesol was cast on a glass Petri dish and dried at ambient temperature inair. The dried gel flakes were recovered from the bottom of the Petridish, and were then heated to 923 K in air at a rate of 1 K/minute andmaintained at this temperature for 2 hours.

The Barrett, Joyner and Halenda (BJH) method was used to determine thepore size distribution using the desorption isotherm. FIG. 9 illustratesthe pore size distributions of the gamma-alumina supports prepared fromthe boehmite sols containing particles of size of 630, 200, and 40 nm.These supports had a sharp pore size distribution. Additionally, Table 1lists the microstructure parameters of these three supports. It wasdiscovered that the larger the particle size of the sols, the larger thepore size and porosity of the resulting membranes, as discussed inconnection with FIG. 1. TABLE 1 Microstructure parameters ofgamma-alumina membranes Gamma-alumina membrane Pore surface Average Solparticle Pore volume area pore size Porosity* size (nm) (cm³ g⁻¹) (m³g⁻¹) (nm) (%) 630 0.4731 370.4 5.11 63.6 200 0.4321 378.7 4.56 61.5 400.3622 388.6 3.73 57.3*ρ_(gamma-alumina) = 3.7 g cm⁻³ (R. S. A. de Lange et al., J. Membr.Sci., 99 (1995) 57)

Silica layers were deposited on the previously described gamma-aluminalayers by a chemical vapor deposition (CVD) method described by Oyama etal. in U.S. Pat. No. 6,527,833, incorporated herein by reference. TheOyama et al. method places a silica layer on a support via thermaldecomposition of tetraethylorthosilicate (TEOS) at high temperature inthe absence of oxygen. While silica/alumina composite membranes arediscussed, any suitable membrane materials may be used including, butare not limited to, alumina, zirconia, titania, silicon nitride, siliconcarbide, boron nitride, perovskites, spinels, pyrochlores, zeolites,metals and the like. Additionally, any acceptable silica-containingspecies may be substituted for TEOS

A suitable CVD apparatus is shown in FIG. 10 and CVD process parametersare listed in Table 2. Briefly, the reactor assembly 210 for depositingthe permselective membrane on the porous substrate 220 comprises anouter concentric, nonporous tubing 230 and a temperature-controlledheater 250. The porous substrate 220 is surrounded by outer concentric,nonporous tubing 203 of larger diameter, forming an annulus (not shown).The inside of the porous tubing 220 is referred to herein as the “tube”side 262 whereas the outside of the porous tubing 220 that is locatedinside the outer concentric, nonporous tubing 230 is referred to as the“shell” side 264. The concentric tubing 220 and outer concentric,nonporous tubing 230 are located in temperature-eater controlled heater250.

A CVD reactant gas is generated by a gas dilution system 310 as depictedin FIG. 10. As used herein, the term CVD reactant gas refers to the gasor gases being deposited in the CVD process. As used herein, the termCVD gas stream refers to the CVD reactant gas as well as any associatedinert carrier gases.

Referring still to FIG. 10, carrier gas from carrier gas cylinder 320passes through a temperature-controlled bubbler 330 containing theliquid CVD material 334. In some embodiments, the carrier gas passesthrough an oxygen-water trap 322 prior to enteringtemperature-controlled bubbler 330. The carrier gas saturated with CVDreactant gas then flows through a tee 340, in which it is mixed withdilution gas flowing from a dilution gas cylinder 350. In someembodiments, the carrier gas saturated with CVD reactant gas passesthrough an oxygen-water trap 342 prior to mixing in dilution gascylinder 350. The mixed gas stream containing the CVD reactant gas,carrier gas and dilution gas then passes into the upstream end 272 ofthe tube side of the porous tubing, through the porous tubing, and outthe downstream end 274 of the tube side of the porous tubing. Theconcentration of the CVD reactant gas in the CVD gas stream can bevaried considerably and accurately by adjusting the temperature of thebubbler 330 as well as the mass flow controllers 312 located downstreamof the carrier gas cylinder 320 and the dilution gas cylinder 350.

The environment surrounding the exterior of the porous tubing 220—i.e.,the shell side 264—is controlled using either a purge gas or a vacuum.When a purge gas is used, the purge gas from purge gas cylinder 360enters through a gas inlet 232 in the outer concentric, nonporous tubing230 and passes out through a gas vent 234 located at the other end ofthe an outer concentric, nonporous tubing 230. In some embodiments, thepurge gas passes through an oxygen-water trap 352 prior to enteringnonporous tubing 230. Alternatively, the environment surrounding ctheexterior of the porous tubing 220 can be evacuated using a vacuum pump(not shown). This is accomplished by connecting a vacuum pump to gasvent 234 and closing the valve just downstream of purge gas cylinder360, thereby establishing an airtight seal.

For the examples, the support covered with the gamma-alumina layers wasinstalled concentrically inside a piece of glass tubing of 14 mm insidediameter (nonporous tubing 230) using machined Swagelok fittings withTeflon ferrules. After placing the assembly in an electrical furnace(temperature controller 250) and heating it to 873 K at a heating rateof 1 K/minute, an argon gas flow (balance gas from gas cylinder 360) wasintroduced on outer shell side 264 and a dilute argon gas flow (dilutegas from gas cylinder 350) was introduced on the inner tube side 262.After 30 minutes a carrier gas flow (carrier gas from gas cylinder 320)was passed through bubbler 330 filled with TEOS (liquid CVD material334) at 296 K and was premixed with the dilute argon gas flow beforeintroduction to the inside of the support. The deposition time wasvaried from 3 to 6 hours. After the CVD process was finished, assembly210 was purged with the balance and dilute gas flows for 30 minutes.

The gas permeation measurement was generally conducted at 873 K on H₂,CH₄, CO and CO₂ by admitting the pure gases at a certain pressure(higher than atmospheric pressure) into the inner tube side, one end ofwhich was closed, and measuring the quantity of gas flowing from theouter tube. The selectivity was calculated as the ratio of thepermeances of 12 to CH₄, CO and CO₂. Permeation of He, H₂, and Ne wasmeasured in a similar manner at different temperatures. TABLE 2 CVDprocess parameters for the preparation of composite membranes Carriergas flow rate (ml min⁻¹) 5.4 Dilute gas flow rate (ml min⁻¹) 19.6Balance gas flow rate (ml min⁻¹) 25.0 TEOS concentration (mol m⁻³)0.0193 CVD temperature (K) 873

Example 1

This example and the following two examples describe the synthesis ofboehmite sols in accordance with a preferred embodiment. It should benoted that the dipping solutions, consisting of the described sols aredilute, not gels. The use of dilute solutions gives rise to thinintermediate layers.

A boehmite sol was prepared by adding 0.2 mol of aluminum isopropoxide(Aldrich, 98+%) to 300 ml of distilled water at room temperature. Themixture was stirred at high speed and heated to 353 K within 30 min. Thealkoxide was hydrolyzed at this temperature for 24 hours, and then thetemperature of the mixture was increased to 365 K, after which the flaskwas opened for 1.5 hours to allow volatilization of the alcohol. Theflask was then closed again and the solution was stirred at 365 K for 1hour with refluxing. A predetermined quantity of nitric acid (VWR,68.0-70.0%), hydrochloric acid (GR, 36.5-38.0%) or acetic acid (GR,99.7%) was added to the solution to give a 0.10 molar ratio ofH⁺/Alkoxide. After peptization at 365 K with refluxing for 20 hours, aclear and stable sol solution was obtained.

FIG. 5 shows the particle size distributions of the three boehmite solsolutions obtained with the corresponding acids. It can be seen that thesols prepared with inorganic acids have smaller average particles sizesthan the sols prepared with acetic acid. The boehmite sol peptized withnitric acid was found to have a median particle size of 55 nm and wasdesignated BS55. BS55 was used for the preparation of the intermediatelayers of gamma-alumina in membrane substrates, later described.

Example 2

Boehmite sols were prepared using the method described in Example 1,except the hydrolyzed solution was peptized with various quantities ofacetic acid to give molar ratios of H⁺/Alkoxide of 0.03, 0.04, 0.055,0.07, 0.10, 0.15 and 0.25. FIG. 6 shows the particle size distributionsin these seven boehmite sol solutions. When acetic acid was used as thepeptizing agent, the median particle size of the resulting boehmite solsincreased from 65 nm to 950 nm with decreasing molar ratio ofH⁺/Alkoxide from 0.25 to 0.03. Thus, lower acetic acid concentrationtends to favor the formation of larger particles. The two boehmite solswhich were peptized with acetic acid at molar ratios of H⁺/Alkoxide of0.04 and 0.07, and which had median particle sizes of 630 and 200 μm,respectively, were designated as BS630 and BS200. BS630 and BS200 werealso used for the preparation of the intermediate layers ofgamma-alumina in membrane substrates, later described.

Example 3

Boehmite sols were prepared using the method described in Example 1,except the aluminum isopropoxide was hydrolyzed at 353 K for varioustimes (0.5, 3, 24 and 72 hours) and the solution was peptized for 20hours with acetic acid at a molar ratio of H⁺/Alkoxide of 0.15. FIG. 7shows the particle size distributions in these boehmite sol solutions.As the hydrolysis time increased from 0.5 hours to 72 h, the medianparticle size of the resulting boehmite sols increased from 13 nm to 120nm. The boehmite sol which was hydrolyzed at 353 K for 3 hours andpeptized with acetic acid at a molar ratio of H⁺/Alkoxide of 0.15 andwhich had a median particle size of 40 nm was designated as BS40. BS40was used for the preparation of the immediate layers of gamma-alumina inmembrane substrates, later described.

Example 4

This example describes the preparation of dipping solutions used in thedipping-calcining procedure for placing alumina layers on top of aporous substrate. The dipping solutions are diluted combinations of thesol solutions described in Examples 1-3 mixed with a binding agent orbinder, polyvinyl alcohol (PVA). It is contemplated that other suitablebinders include, but are not limited to, polysaccharide, starch, stearicacid, polylactic acid, polymethylmethacrylate, polysulfone, polyimide,or any polymer or polyelectrolyte-containing polar groups.

Several dipping solutions with sol concentrations of 0.15 M wereprepared. The dipping solution made using the boehmite sol BS55 withmedian particle size of 55 nm described in Example 1 was designated asDS55. DS55 was prepared by adding 3.5 g of PVA (Fluka, M.W.=72,000) and5 ml of 1 M HNO₃ into 95 ml of boiling water with vigorous stirring andrefluxing. After 4 hours, a clear solution with a PVA concentration of3.5 weight percent was obtained. Then, 37.5 ml of 0.80 M boehmite solBS55 was vigorously mixed with 142 ml of distilled water and 20 ml ofthe 3.5 weight percent PVA solution at above 323 K for 2 hours withrefluxing. The final concentrations of PVA and boehmite in the sol were0.35 weight percent and 0.15 M, respectively. The solution was cooled toroom temperature for 1 hour without stirring and was set aside for thepreparation of the gamma-alumina membranes.

Dipping solutions DS630, DS200 and DS40 were obtained by the sameprocedure using the boehmite sols BS630, BS200 and BS40 described inExamples 2 and 3. These boehmite sols had median particle sizes of 630,200 and 40 nm, respectively.

Example 5

This comparative example describes the preparation of ungraded membranesubstrates by the deposition of intermediate gamma-alumina layers on topof a porous support, where the layers are formed from a single dippingsolution. The support used was a commercial alumina membrane tube, 3 cmlong with a nominal pore size of 100 nm.

A dip-coating method was employed to coat the alumina supports with thesol and binder materials. First, dipping solution DS55 containing theboehmite sol with median particle size of 55 nm described in Example 4was used. The support was dipped at a speed of 0.01 meter/second indipping solution DS55, was held for 10 seconds, and was withdrawn at thesame speed. The sol-coated tube was dried in ambient air for 24 h,heated up to 923 K at a heating rate of 1 K/minute and calcined for 2hours. This dipping-calcining procedure was repeated 4 times and theresulting membrane substrate was designated as membrane substrate55-55-55-55. The permeation properties of membrane substrate 55-55-55-55are listed in Table 3.

A similar procedure was used to prepare a membrane substrate usingdipping solution DS630. This was designated membrane substrate630-630-630-630. The permeation properties of membrane substrate630-630-630-630 are also listed in Table 3. TABLE 3 Permeationproperties of Example 5 Membrane substrate Membrane substrate Permeationproperties 55-55-55-55 630-630-630-630 Permeance H₂ 4.8 × 10⁻⁵ 6.9 ×10⁻⁵ (mol/m² s Pa) CH₄ 1.8 × 10⁻⁵ 2.6 × 10⁻⁵ CO₂ 1.1 × 10⁻⁵ 1.5 × 10⁻⁵Selectivity H₂/CH₄ 2.7 2.7 H₂/CO₂ 4.4 4.6

Example 6

This example describes the preparation of graded membrane substrates forthe formation of composite membranes. The membrane substrates were madeby the deposition of intermediate gamma-alumina layers on top of aporous support, where the layers were formed from dipping solutionshaving sequentially smaller average particle sizes. In some embodiments,the dipping-calcining steps were repeated.

The same method was used as in Example 5, except the fourdipping-calcining steps were carried out with different 0.15 M dippingsolutions in the order DS630, DS630, DS200 and DS40. As described inExample 4, the solutions DS630, DS200 and DS40 contained the sols withmedian particle sizes of 630, 200 and 40 nm, respectively. First, thesupport was dipped in the dipping solution DS630, dried and thencalcined as described in Example 5. Then, the dipping-calciningprocedure was repeated using the same dipping solution DS630, followedby application of the dipping-calcining procedure with solutions DS200,and DS40. This membrane substrate was designated membrane substrate630-630-200-40. The pore size distribution of membrane substrate630-630-200-40 is given in FIG. 9 and the permeation properties arelisted in Table 4.

The same procedure was followed to deposit five layers in the orderDS630, DS630, DS200, DS40 and DS40. The resulting membrane substrate wasdesignated membrane substrate 630-630-200-40-40 and its permeationproperties are also listed in Table 4. TABLE 4 Permeation properties ofExample 6 Membrane substrate Membrane substrate Permeation properties630-630-200-40 630-630-200-40-40 Permeance H₂ 5.9 × 10⁻⁵ 4.5 × 10⁻⁵(mol/m² s Pa) CH₄ 2.2 × 10⁻⁵ 1.6 × 10⁻⁵ CO₂ 1.3 × 10⁻⁵ 9.1 × 10⁻⁶Selectivity H₂/CH₄ 2.7 2.8 H₂/CO₂ 4.5 4.9

Example 7

This comparative example describes the preparation of compositesilica/alumina membranes employing ungraded membrane supports assubstrates. The composite membranes consist of thin silica layersdeposited by CVD on top of the sol-treated substrates obtained inExample 5 (i.e. 55-55-55-55 and 630-630-630-630). The silica layer wasdeposited by the CVD of TEOS at high temperature as described in U.S.Pat. No. 6,527,833 and in the “Examples Overview” section.

The CVD process was conducted for 3 hours and 6 hours with the apparatusshown in FIG. 10 and the process parameters listed in Table 2. Thepermeation properties of the resulting composite membranes at 873 Kbefore and after CVD are listed in Tables 5 and 6. TABLE 5 Gaspermeation properties of composite membrane 55-55-55-55 Membranesubstrate Permeation properties 55-55-55-55 3 hr-SiO₂ 6 hr-SiO₂Permeance H₂ 4.8 × 10⁻⁵ 1.1 × 10⁻⁶ 9.4 × 10⁻⁷ (mol/m² s Pa) CH₄ 1.8 ×10⁻⁵ 4.1 × 10⁻⁷ 3.1 × 10⁻⁷ CO₂ 1.1 × 10⁻⁵ 3.2 × 10⁻⁷ 2.0 × 10⁻⁷Selectivity H₂/CH₄ 2.7 2.7 3.0 H₂/CO₂ 4.4 4.7 4.7

TABLE 6 Gas permeation properties of composite membrane 630-630-630-630Membrane substrate Permeation properties 630-630-630-630 3 hr-SiO₂ 6hr-SiO₂ Permeance H₂ 6.9 × 10⁻⁵ 1.6 × 10⁻⁶ 4.6 × 10⁻⁷ (mol/m² s Pa) CH₄2.6 × 10⁻⁵ 4.8 × 10⁻⁷ 6.9 × 10⁻⁸ CO₂ 1.5 × 10⁻⁵ 3.0 × 10⁻⁷ 3.3 × 10⁻⁸Selectivity H₂/CH₄ 2.7 3.3 6.7 H₂/CO₂ 4.6 5.3 14  

Looking at Tables 5 and 6, the selectivities of H₂ over CH₄ and CO₂ forthe substrate membrane with intermediate layers of gamma-alumina areclose to the values predicated by the Knudsen diffusion mechanism. Forexample, after 3 hours and 6 hours of CVD, the permeabilities declinedslightly while the selectivities increased slightly. In contrast, the H₂permeance was high, 9.4×10⁻⁷ mol Pa⁻¹ s⁻¹ m⁻².

Example 8

This example describes the preparation of a silica membrane utilizing amembrane substrate with four multiple graded layers. The membraneconsists of a thin silica layer deposited by CVD on top of a membranesubstrate obtained in Example 6 (i.e. using dipping solutions DS630,DS630, DS200 and DS40).

The silica layer was deposited by the CVD of TEOS at high temperature asdescribed in U.S. Pat. No. 6,527,833, in a similar manner as in Example7. The CVD process was conducted for 5 hours with the process parameterslisted in Table 2. The permeation properties of the resulting membraneat 873 K before and after CVD are listed in Table 7 and are graphicallyshown in FIG. 12. TABLE 7 Gas permeation properties of compositemembrane 630-630-200-40 Membrane substrate Permeation properties630-630-200-40 5 hr-SiO₂ Permeance H₂ 5.9 × 10⁻⁵ 1.9 × 10⁻⁷   (mol/m² sPa) CH₄ 2.2 × 10⁻⁵ 2.9 × 10⁻¹⁰ CO 1.7 × 10⁻⁵ 3.2 × 10⁻¹⁰ CO₂ 1.3 × 10⁻⁵3.5 × 10⁻¹⁰ Selectivity H₂/CH₄ 2.7 660 H₂/CO 3.5 590 H₂/CO₂ 4.5 540

Looking at Table 7, the selectivities of the membrane increased rapidlywith the deposition time. For example, after 5 hours of deposition, theselectivities of H₂ over CH₄, CO and CO₂ were 660, 590 and 540,respectively at 873 K. Also, the H₂ permeance was 1.9×10⁷ mol Pa⁻¹ s⁻¹m⁻², clearly superior to that with the ungraded substrates of Examples 6and 7.

Example 9

This example describes the preparation of another silica membraneprepared using a membrane substrate with five graded layers. Themembrane had a thin silica surface layer deposited by CVD, similarly toExamples 7 and 8.

The substrate in this case was one of the membrane substrates describedin Example 6 which had five graded layers of different particle-sizedgamma-alumina on a porous alumina tube (i.e. using sequentially dippingsolutions DS630, DS630, DS200, DS40 and DS40). The CVD of TEOS wasconducted for 3 hours with the process parameters listed in Table 2. Thepermeation properties of the resulting membrane at 873 K before andafter CVD are listed in Table 8 and graphically shown in FIG. 13. TABLE8 Gas permeation properties of composite membrane 630-630-200-40-40Membrane substrate Permeation properties 630-630-200-40-40 3 hr-SiO₂Permeance H₂ 4.5 × 10⁻⁵ 4.9 × 10⁻⁷   (mol/m² s Pa) CH₄ 1.6 × 10⁻⁵ 8.3 ×10⁻¹¹ CO 1.2 × 10⁻⁵ 9.6 × 10⁻¹¹ CO₂ 9.1 × 10⁻⁶ 3.2 × 10⁻¹⁰ SelectivityH₂/CH₄ 2.8 5900 H₂/CO 3.7 5100 H₂/CO₂ 4.9 1500

Looking at Table 8, the selectivities of the membrane increased quicklyafter the deposition. For example, after 3 hours of deposition, theselectivities of H₂ over CH₄, CO and CO₂ were 5900, 5100 and 1500,respectively at 873 K. The H₂ permeance was 4.9×10⁷ mol m⁻² s⁻¹ Pa⁻¹,which was significantly better than that of the four-layer gradedsubstrate of Example 8.

Example 10

This example illustrates the unique permeability properties of a silicamembrane using graded substrates in accordance with a preferredembodiment of this invention. A composite membrane with a five-layersubstrate similar to the one described in Example 9 was prepared withfreshly synthesized sols as described in Examples 1 and 2. The medianparticle sizes of the sols were 540, 540, 170, 40, 40 nm, producingmembrane 540-540-170-40-40.

The permeability of the membrane for He, H₂, and Ne was measured and theresults are presented in FIG. 14. In FIG. 14, the permeability of themembrane rises with temperature, as expected. The almost completeexclusion of CO₂, CO, and CH₄ (Table 8) can be understood from the largesize (>0.3 nm) of these species (see Table 9). TABLE 9 Size and weightof permeating species Species Size/nm Weight/atomic units He 0.260 4 Ne0.275 20 H₂ 0.289 2 CO₂ 0.33 44 CO 0.376 28 CH₄ 0.38 16

Looking still at FIG. 14, the permeation properties of the membraneappear to be those of the silica layer, as it was already demonstratedthat the porous substrate permitted the permeation of the larger speciesin Examples 7-9.

However, the order of permeability, He>H₂>Ne, is unexpected because itfollows neither the size nor weight of the species; generally, porousmaterials allow faster passage of smaller species or lighter species.Comparing Table 9 with FIG. 14, He is heavier than H₂, yet permeatesfaster. Ne is smaller than H₂, yet permeates slower. Without wishing tobe bound by any particular theory, the order and behavior of the speciesappears to be explained by a solubility site mechanism for permeationoriginally derived to describe permeation in vitreous glasses (J. S.Masaryk, R. M. Fulrath, J. Chem. Phys. 59 (1973) 1198). The governingequation for the solubility site mechanism is: $\begin{matrix}{Q = {\frac{1}{6L}\left( \frac{\mathbb{d}^{2}}{h} \right)\left( \frac{h^{2}}{2\pi\quad{mkT}} \right)^{\frac{3}{2}}\frac{\left( {N_{S}/N_{A}} \right)}{\left( {{\mathbb{e}}^{{{hv}^{*}/2}{kT}} - {\mathbb{e}}^{{{- {hv}^{*}}/2}{kT}}} \right)^{2}}{\mathbb{e}}^{{- \Delta}\quad{E_{K}/{RT}}}}} & \lbrack 4\rbrack\end{matrix}$

In Equation 4, Q=permeability, L=membrane thickness, d=jump distance,h=Planck's constant, m=mass of permeating species, k=Boltzmann constant,T=absolute temperature, Ns=number of solubility sites, N_(A)=Avogadro'snumber, ν=jump frequency, ΔE_(K)=activation energy, R=gas constant. Theequation above is simplified in that it does not consider loss ofrotation of the hydrogen molecule. The calculated curves, assuming ajump distance of 0.8 μm, fit the experimental points very well (FIG.14). The calculated parameters are summarized in Table 10. TABLE 10Calculated Parameters for Composite Membrane 540-540-170-40-40 KineticGases Diameter (nm) N_(s) (sites m⁻³) v* (s⁻¹) E_(a) (kJ mol⁻¹) He 0.266.79 × 10²⁶ 8.59 × 10¹² 4.07 H₂ 0.289 4.01 × 10²⁶ 1.13 × 10¹³ 8.90 Ne0.275 5.00 × 10²⁶ 4.40 × 10¹² 8.75

Generally, the number of solubility sites is larger for the smallerspecies, as on the average there will be more sites available toaccommodate smaller sized species. The jump frequencies are inverselyproportional to the molecular weight of the species, as lighter speciesvibrate faster in their equilibrium sites. The size of the solubilitysites of the silica layer is smaller than 0.3 nm, because CO₂, CO andCH₄ do not permeate. Thus, the silica layer limits the permeation of themembrane to species smaller than 0.3 nm.

The results obtained can be compared to values for vitreous glass wherethe jump distance is 0.3 nm, shown in Table 11. TABLE 11 Parameters forVitreous Glass Gases N_(s) (sites m⁻³) v* (s⁻¹) E_(a) (kJ mol⁻¹) He 2.22× 10²⁷ 6.90 × 10¹² 17.8-21.1 H₂ 1.07 × 10²⁷ 1.22 × 10¹³ 37.2-38.3 Ne1.30 × 10²⁷ 4.38 × 10¹² 33.8-39.5

Compared to vitreous glass, the number of solubility sites in compositemembrane 540-540-170-40-40 are smaller, consistent with the larger jumpdistance and a more open structure. The jump frequencies are similar,indicating a similar environment as in vitreous glass. The activationenergies in the membrane are considerably smaller, again indicating thatthe structure of the silica is more open and less restrictive than thatof the vitreous glass.

FIGS. 15 and 16, respectively, are low- and high-resolution electronmicrographs of exemplary five-layer composite membrane630-630-200-40-40. Referring first to FIG. 15, the bottom portion of themicrograph shows the coarse structure of the porous support, while thetop portion shows the 5-layer substrate. The boundary between layers isnot discernible, partly because only 3 sol sizes (630, 200, 40) wereused. Close inspection shows the particles at the top are smaller thanthose at the bottom.

Referring now to FIG. 16, the micrograph show the topmost part of themembrane formed by the sols of size 40 nm. Careful examination reveals athin layer at the very top with a slightly different contrast than thealumina particles. This is the silica layer, of thickness 10-20 nm.

While the preferred embodiments of the present invention have been shownand described, modifications thereof can be made by one skilled in theart without departing from the spirit and teachings of the invention.The embodiments described herein are exemplary only, and are notintended to be limiting. Many variations and modifications of theinvention disclosed herein are possible and are within the scope of theinvention. For example, while the sol solution was described as beingapplied to a support via dipping, it is contemplated that any method ofcoating, including, but not limited to, painting, spraying, rolling,etc., may be used.

Accordingly, the scope of protection is not limited by the descriptionset out above, but is only limited by the claims which follow, thatscope including all equivalents of the subject matter of the claims.Each and every claim is incorporated into the specification as anembodiment of the present invention. Thus the claims are a furtherdescription and are an addition to the preferred embodiments of thepresent invention. Use of the term “optional” with respect to anyelement of a claim is intended to mean that the subject element isrequired, or alternatively, is not required. Both alternatives areintended to be within the scope of the claim. The discussion of areference in the Description of Related Art is not an admission that itis prior art to the present invention, especially any reference that mayhave a publication date after the priority date of this application. Thedisclosures of all patents, patent applications and publications citedherein are hereby incorporated herein by reference, to the extent thatthey provide exemplary, procedural or other details supplementary tothose set forth herein.

1. A permselective membrane assembly comprising a porous, gradedparticulate layer.
 2. The permselective membrane assembly of claim 1further comprising a porous support and a permselective membrane.
 3. Thepermselective membrane assembly of claim 2 wherein the porous support isselected from the group consisting of alumina, titania, silica,zirconia, perovskites, spinels, pyrochlores, zeolites, stainless steel,and combinations thereof.
 4. The permselective membrane assembly ofclaim 3 wherein the porous support comprises alumina.
 5. Thepermselective membrane assembly of claim 2 wherein the pernselectivemembrane is deposited by CVD.
 6. The permselective membrane assembly ofclaim 2 wherein the permselective membrane comprises silica.
 7. Thepermselective membrane assembly of claim 2 wherein the permselectivemembrane comprises nanosil.
 8. The permselective membrane assembly ofclaim 1 wherein the permeance to hydrogen gas is at least 1×10−7 mol/m²s Pa and the hydrogen permselectivities with respect to carbon dioxide,carbon monoxide and methane are each at least
 100. 9. The permselectivemembrane assembly of claim 1 wherein the permeance to hydrogen gas is atleast 4×10−7 mol/m² s Pa and the hydrogen permselectivities with respectto carbon dioxide, carbon monoxide and methane are each at least 800.10. The permselective membrane assembly of claim 1 wherein the porous,graded particulate layer is prepared from a plurality of sols havingnarrow particle size distributions.
 11. The permselective membrane ofclaim 10 wherein a least one of the sols has an average particle size ofgreater than 400 nm and at least one of the sols has an average particlesize less than 100 nm.
 12. The permselective membrane of claim 10wherein the plurality of sols are comprised of alumina particles.
 13. Amethod for promoting the yield of a gaseous reaction product byselective permeation using a permselective membrane assembly comprisinga porous, graded particulate layer.
 14. The method of claim 13 whereinthe gaseous reaction product is hydrogen.
 15. A sol comprised of aluminaparticles having a median particle size of at least 50+/−25 nm.
 16. Asol comprised of alumina particles having a median particle size of atleast 200+/−75 nm.
 17. A sol comprised of alumina particles having amedian particle size of at least 600+/−200 rm.
 18. A method forproducing a stable boehmite sol having a narrow particle distributioncomprising; hydrolyzing an aluminum-containing precursor at conditionssufficient to form an aluminum-containing intermediate; peptizing thealuminum-containing intermediate with an acid at conditions sufficientto form a colloid; and aging the colloid at conditions sufficient toform a stable sol.
 19. The method of claim 18 wherein thealuminum-containing precursor is selected from the group consisting ofaluminum alkoxides, aluminum chlorides, and aluminum isopropoxides. 20.The method of claim 18 wherein the hydrolysis step is performed attemperatures in the range of 340 K and 370 K.
 21. The method of claim 20wherein the hydrolyzation step is performed at times in the range of 0.5hours and 72 hours.
 22. The method of claim 18 wherein the acid isselected from the group consisting of nitric acid, hydrochloric acid,acetic acid, citric acid, or tartaric acid or combinations thereof. 23.The method of claim 18 wherein the peptization step is performed at amolar ratio of H⁺/Alkoxide in the range of 0.03 and 0.25.
 24. The methodof claim 23 wherein the peptization step is performed at times in therange of 1 hours and 20 hours.
 25. A boehmite sol produced by the methodof claim
 18. 26. A method for preparing an aluminum-containing solutioncomprising mixing a boehmite sol with a binder-containing solution,wherein the boehmite sol is produced by a method comprising: hydrolyzingan aluminum-containing precursor at conditions sufficient to form analuminum-containing intermediate; peptizing the aluminum-containingintermediate with an acid at conditions sufficient to form a colloid;and aging the colloid at conditions sufficient to form the sol.
 27. Themethod of claim 26 wherein the binder is selected from the groupconsisting of polymers and polyelectrolytes.
 28. The method of claim 27wherein the binder is a polymeric binder selected from the groupconsisting of polyvinyl alcohol, polysaccharide, starch, stearic acid,polylactic acid, polymethylmethacrylate, polysulfone, polyimide,polyvinyl pyrolidone, polyacrylamides, hydroxyethylcellulose, guar gumsor other water-soluble polymers.
 29. The method of claim 28 wherein thepolymeric binder is polyvinyl alcohol.
 30. The method of claim 29wherein the aluminum-containing solution comprises polyvinyl alcohol inthe range of 0.035 wt % and 3.5 wt %.
 31. The method of claim 30 whereinthe aluminum-containing solution comprises boehmite sol in the range of0.015 and 1.5 M.
 32. An aluminum-containing solution produced by themethod of claim
 26. 33. A method for forming a porous membranecomprising: coating a porous support with an aluminum-containingsolution having a first particle size; and calcining the support atconditions sufficient to convert the aluminum-containing solution to agamma-alumina layer; wherein the aluminum-containing solution comprisesa boehmite sol and a binder-containing solution and wherein the boehmitesol is produced by a method comprising: hydrolyzing analuminum-containing precursor at conditions sufficient to form analuminum-containing intermediate; peptizing the aluminum-containingintermediate with an acid at conditions sufficient to form a colloid;and aging the colloid at conditions sufficient to form the sol.
 34. Themethod of claim 33 wherein the coating and calcining steps are repeatedat least once using an aluminum-containing solution having a secondparticle size, the second particle size being smaller than the firstparticle size.
 35. The method of claim 34 wherein the porous support isselected from the group consisting of alumina, titania, silica,zirconia, perovskites, spinels, pyrochlores, zeolites, or anycombination thereof.
 36. The method of claim 35 wherein the poroussupport is alumina.
 37. The method of claim 34 wherein the coating andcalcining steps are repeated four times.
 38. The method of claim 34wherein the gamma-alumina layer has a thickness in the range of 200 μmand 2,000 nm.
 39. The method of claim 38 wherein the gamma-alumina layerhas a thickness in the range of 400 nm and 1,800 nm.
 40. The method ofclaim 39 wherein the gamma-alumina layer has a thickness in the range of500 μm and 1,500 nm.
 41. A porous membrane produced by the method ofclaim
 33. 42. A method for preparing a composite membrane comprising:coating a porous support with an aluminum-containing solution having afirst particle size; calcining the support at conditions sufficient toconvert the aluminum-containing solution to a gamma-alumina layer; anddepositing a silica layer on top of the gamma-alumina layer; wherein thealuminum-containing solution comprises a boehmite sol and abinder-containing solution and wherein the boehmite sol is produced by amethod comprising: hydrolyzing an aluminum-containing precursor atconditions sufficient to form an aluminum-containing intermediate;peptizing the aluminum-containing intermediate with an acid atconditions sufficient to form a colloid; and aging the colloid atconditions sufficient to form the sol.
 43. The method of claim 42wherein the silica layer is deposited on the gamma-alumina layer bychemical vapor deposition.
 44. The method of claim 42 wherein the silicalayer has a thickness in the range of 10 nm and 50 nm.
 45. The method ofclaim 42 wherein the silica layer has solubility sites of less than 0.3nm size.
 46. A composite membrane produced by the method of claim 42.